Tractors used to carry a metal working tool along a fixed path on a workpiece, their operation over the associated track are adequately shown and described in U.S. Pat. No. Re. 28,121 granted Aug. 20, 1974. The Arcair Company of Lancaster, Ohio offers an all position, multipurpose remote controlled tractor of this type under the trademark CLIMBER.
Another widely used device is that shown in U.S. Pat. No. 2,474,153. The apparatus of the '153 patent employs four guiding wheels with a tapered cross-section. The thinnest part of the tapered wheel is at the largest diameter of the wheel, the wheels adapted to mate with a complimentary shape on a track fixed to a workpiece.
In addition to the foregoing, a method of mounting a track to the workpiece is shown in U.S. Pat. No. 3,226,027.
U.S. Pat. Nos. 3,176,587 and 3,259,021 disclose attempts to improve a tool carrying device by using a three wheeled configuration. The third wheel of these devices is directly coupled to the motor and is permitted to pivot about a point to compensate for changes in curvature in the track, misalignment of joints and the like.